1. Field of the Invention
The present invention relates to a steel wire rod having excellent drawability and fatigue properties and a manufacturing method of the wire rod, and particularly relates to a steel wire rod in which non-metallic inclusions that are hard and have extremely small ductility are decreased and thus drawability and fatigue properties are improved, and a useful method for manufacturing the steel wire rod.
2. Description of Related Art
When the non-metallic inclusions that are hard and have extremely small ductility (particularly oxide base inclusions, hereinafter, sometimes called merely “inclusions”) exist in a steel wire rod, the non-metallic inclusions may cause breakage in a process of drawing the rod to an ultra-fine steel wire such as a tire cord. Moreover, when the steel wire rod is used for spring manufacturing, fatigue fracture may start from the non-metallic inclusions in a condition that repeated stress is loaded to obtained products (springs). Therefore, it is essential that the non-metallic inclusions are reduced to the utmost or softened to increase ductility in order to eliminate adverse influence of the inclusions in a manufacturing process of the steel wire rods.
In the light of improving softness and ductility of the non-metallic inclusions in the steel wire rod, various techniques have been proposed. For example, U.S. Pat. No. 6,328,820, JP-A No. 2003-49244 and JP-B No. 6-74485 show methods of improving softness and ductility of the inclusions by controlling a composition of the non-metallic inclusions in steel to be within a certain range. Specifically, U.S. Pat. No. 6,328,820 shows that the number of oxide base inclusions 5 μm or less in thickness is controlled to be within a certain range with respect to all oxide base inclusions in an L-section of rolled steel, thereby certain fatigue properties can be ensured. However, the number of oxide base inclusions 5 μm or less in thickness is specified only in a ratio of at least 80% in the L-section of the rolled steel, and further improvement is considered to be required for securely improving the fatigue properties.
JP-A No. 2003-49244 specifies that among non-metallic inclusions detected in the L-section of the rolled steel, at least 80% of non-metallic inclusions, in which a ratio L/D of the major axis (L) to the minor axis (D) is more than 5, and D is 10 μm or more, include 10 to 40% of CaO, 30 to 50% of SiO2, 1 to 5% of MnO, 1 to 10% of Al2O3, and 5 to 20% of Na2O. However, the technique is directed to only coarse inclusions as control objects, and the above composition is shown only in an average composition of existing non-metallic inclusions. Consequently, further investigation is necessary for securely improving drawability and the like.
JP-B No. 6-74485 discloses a high cleanliness steel having excellent cold workability and fatigue properties, including non-metallic inclusions having a ratio l/d≦5 of length (l) to width (d) in the L-section of the rolled steel, of which the average composition is 30 to 50% of SiO2, 1 to 10% of Al2O3, 50% or less of CaO, and 50% or less of MgO. However, again in the technique, the composition of the non-metallic inclusions is controlled only in the average composition, and therefore secure improvement in fatigue properties is considered to be difficult.
On the other hand, JP-A No. S53-76916, JP-A No. H4-272119, JP-A No. 2000-212636, JP-A No. H10-102132, and the 182nd, 183rd Nishiyama Memorial Technical Seminar “Inclusion Control and Material Manufacturing Technology of High Cleanliness Steel”, edited by The Iron and Steel Institute of Japan 2004, p 138, show methods that a slag composition is controlled in a certain range in molten steel refining, and molten steel and slag are stirred to be contacted and mixed in order to reform inclusions to be soft and ductile. While a method for contacting the molten steel and the slag is considered to be also important for inclusion control, JP-A No. S53-76916 and JP-A No. H4-272119 do not specifically show such a method. The “Inclusion Control and Material Manufacturing Technology of High Cleanliness Steel” show a phenomenon that a slag having 0.8 to 1.2 of CaO/SiO2 in a CaO/SiO2 system is used for molten steel treatment, thereby non-ductile inclusions are decreased. However, when a molten steel treatment method, that is, a contact mixing method of slag and molten steel is not appropriate, the inclusions are hardly decreased sufficiently. While JP-A No. 2000-212636 and JP-A No. H10-102132 describe control of a rate of blowing gas during refining, in either case the gas rate is high, and therefore inclusions due to the slag are considered to be easily produced.